1. Field of the Invention
The present invention relates to a method, device, and system for regeneration and application of an optical clock.
2. Description of the Related Art
This application is related to U.S. application Ser. Nos. 09/217,018, 09/386,847 and 09/456,821, which are incorporated herein by reference.
A mode-locked laser (MLL) is one of the most general high-quality laser light sources, and it is broadly classified into an active mode-locked laser and a passive mode-locked laser. The active mode-locked laser is of a type such that amplitude modulation (AM) or frequency modulation (FM) is added under laser oscillation conditions to thereby oscillate pulse light synchronous with the modulation. On the other hand, the passive mode-locked laser is of a type such that such modulation is not added, but external pulse light is input to generate pulse light having a shorter pulse duration in response to the external input pulse light. Each type of mode-locked laser is most generally used as a short-pulse light source.
The present invention provides an active mode-locked laser in which external signal light (general signal light used in optical communication) is added to oscillate continuous pulse light synchronous with a fundamental frequency (modulation frequency) of the signal light. The continuous pulse light is output as an optical clock. Thus, the active mode-locked laser according to the present invention can be used not only as a high-quality pulse light source, but also as an optical clock regenerator in an optical repeater.
Conventionally known is a device for regenerating an optical clock so configured that input signal light is once converted into an electrical signal by a photodetector such as a photodiode, that a fundamental frequency is electrically extracted according to the converted electrical signal, and that laser light is intensity-modulated at this fundamental frequency to thereby obtain the optical clock. This kind of device is applied to a regenerative repeater in optical fiber communication, for example. However, the operating speed of this device is limited by an electrical circuit related to signal processing, so that the bit rate of applicable input signal light is limited by the electrical circuit.
Further, another optical clock regenerator employing an active mode-locked laser with an optical modulator (LN (lithium niobate) modulator, EA (electroabsorption) modulator, etc.) inserted therein is also known. In this optical clock regenerator, the optical modulator is modulated at a fundamental frequency electrically regenerated as similarly to the above to regenerate clock pulses.
In any prior art technique mentioned above, it is necessary to perform opto/electric conversion such that signal light is converted into an electrical signal to regenerate an optical clock, and the operation of the device is dependent on the bit rate, pulse shape, etc. of signal light.